Servo system for following a magnetic track having identifying pulse widths

Abstract

A servo system positions a read head for following a magnetic tape track by determining position information from a composite signal read from the tape by the read head. The servo system includes the read head, a discriminator circuit that provides a drive signal in response to the composite signal, and a head positioner that moves the read head in response to the drive signal. One frame of the composite signal in one embodiment includes four sync pulses followed by four position pulses. Sync pulses have a pulse width less than the pulse width of position pulses. The discriminator includes range check circuits that validate sync and position pulses and includes an arithmetic circuit that determines a ratio of sync-to-position time divided by sync-to-sync time. The discriminator provides the drive signal in response to a moving average of such ratios.

What is claimed is: 
     
       1. A head for recording a flux pattern onto a magnetic tape, the tape having a center line along the length of the tape and a plurality of tracks parallel to the center line, the flux pattern for providing indicia for positioning a provided read head during operation of the read head, the head comprising: a. a first flux gap that extends across the plurality of tracks, the first gap having a first gap length along the center line, the first gap operative to provide a rectangular flux pattern in each track thereby providing a first pulse having a first pulse width as the flux pattern is read by the read head from any track of the plurality of tracks; and   b. a second flux gap that extends across the plurality of tracks, the second gap having a second gap length along the center line that differs from the first gap length, wherein operating the head creates flux in the first gap and in the second gap, the second gap operative to provide a parallelogram-shaped flux pattern in each track thereby providing a second pulse having a second pulse width as the flux pattern is read by the read head from any track of the plurality of tracks, the second pulse width differs from the first pulse width, and the indicia for positioning comprise a time between the first pulse and the second pulse.   
     
     
       2. The head of claim 1 wherein the first gap length is smaller than the second gap length. 
     
     
       3. The head of claim 1 wherein a minimum distance between the first gap and the second gap along a line parallel to the center line exceeds the first gap length and exceeds the second gap length. 
     
     
       4. The head of claim 1 wherein the first gap crosses the center line at a right angle and the second gap extends in a zigzag across the plurality of tracks. 
     
     
       5. A method for recording onto a tape a regularly spaced series of magnetized regions for providing indicia for positioning a read head during operation of the read head, the tape having a center line along the length of the tape and a plurality of tracks, the method comprising, in sequence: a. providing a head wherein: (1) the head comprises a first gap that extends across the plurality of tracks, the first gap having a first length along the center line;   (2) the head further comprising a second gap that extends across the plurality of tracks, the second gap having a second length along the center line that differs from the first length; and   (3) a minimum distance between the first gap and the second gap along a line parallel to the center line exceeds the first length and exceeds the second length; and     b. operating the head to produce flux in the first gap and in the second gap;   c. moving the tape along the center line a first distance less than the minimum distance;   d. operating the head to produce flux in the first gap and in the second gap;   e. moving the tape along the center line a second distance; and   f. repeating steps b. through e. wherein the first gap in operation provides a rectangular flux pattern in each track thereby providing a first pulse having a first pulse width as the flux pattern is read by the read head from any track of the plurality of tracks, the second gap in operation provides a parallelogram-shaped flux pattern in each track thereby providing a second pulse having a second pulse width as the flux pattern is read by the read head from any track of the plurality of tracks, the second pulse width differs from the first pulse width, and the indicia for positioning comprise a time between the first pulse and the second pulse.   
     
     
       6. The method of claim 5 wherein the tracks are parallel to the center line. 
     
     
       7. A servo system for following a magnetic tape track, the track having a center line, the system comprising: a. a head that provides a composite signal in response to flux reversals in the track, the composite signal comprising a plurality of pulses, the plurality comprising: (1) a first signal having a first pulse width;   (2) a second signal having a second pulse width that differs from the first pulse width; and   (3) a third signal having a third pulse width that differs from the second pulse width;     b. a discriminator circuit for providing a drive signal in response to the composite signal, the discriminator circuit comprising: (1) a first range check circuit that provides a first sync signal when the first pulse width is within a first range and provides a second sync signal when the third pulse width is within the first range;   (2) a timing circuit that identifies a first time in response to the first sync signal, identifies a second time in response to detecting occurrence of the second signal in response to determining that a pulse width of a pulse of the plurality of pulses differs from the first pulse width, and identifies a third time in response to the second sync signal; and   (3) an arithmetic circuit that determines a first duration in response to the first time and the second time, determines a second duration in response to the first time and the third time, determines a ratio comprising the first duration divided by the second duration, and provides the drive signal in response to the ratio; and     c. a head positioner that moves the head toward the center line in response to the drive signal.   
     
     
       8. The servo system of claim 7 wherein the arithmetic circuit provides the drive signal in further response to a moving average of a plurality of values of the ratio. 
     
     
       9. The servo system of claim 7 wherein the arithmetic circuit further determines a difference between a reference value and the ratio and provides the drive signal in response to the difference. 
     
     
       10. The servo system of claim 7 wherein: a. the arithmetic circuit further determines a first range signal in response to the second duration; and   b. the first range check circuit identifies the first range in response to the first range signal.   
     
     
       11. The servo system of claim 10 wherein the first range check circuit comprises: a. an edge detect circuit that provides, in response to the first signal, a leading edge signal and a trailing edge signal;   b. a clock circuit that provides a clock signal;   c. a register that stores the first range in response to the first range signal;   d. a counter circuit responsive to the leading edge signal for receiving the first range as a starting value and responsive to the trailing edge signal for ceasing counting thereby determining a terminal value; and   e. a comparator circuit that provides the sync signal in response to the terminal value.   
     
     
       12. The servo system of claim 7 wherein: a. the first signal comprises a positive pulse and a negative pulse; and   b. the first range check circuit provides the first sync signal when the positive pulse is characterized by the first pulse width within the first range and the negative pulse is characterized by the first pulse width within the first range.   
     
     
       13. The servo system of claim 7 wherein: a. the first signal comprises in sequence a first pulse having a first polarity, a second pulse having a second polarity opposite the first polarity, a third pulse having the first polarity, and a fourth pulse having the second polarity; and   b. the first range check circuit provides the first sync signal when each pulse of the set consisting of the first pulse, the second pulse, the third pulse, and the fourth pulse, is characterized by the first pulse width within the first range.   
     
     
       14. The servo system of claim 7 wherein: a. the discriminator circuit further comprises a second range check circuit that provides a position signal in response to the second signal when the second pulse width is within a second range; and   b. the timing circuit identifies the second time in further response to the position signal.   
     
     
       15. The servo system of claim 14 wherein: a. the arithmetic circuit further determines a second range signal in response to the second duration; and   b. the second range check circuit identifies the second range in response to the second range signal.   
     
     
       16. The servo system of claim 14 wherein: a. the second signal comprises a positive pulse and a negative pulse; and   b. the second range check circuit provides the position signal when the positive pulse is characterized by the second pulse width within the second range and the negative pulse is characterized by the second pulse width within the second range.   
     
     
       17. The servo system of claim 14 wherein: a. the second signal comprises in sequence a first pulse having a first polarity, a second pulse having a second polarity opposite the first polarity, a third pulse having the first polarity, and a fourth pulse having the second polarity; and   b. the second range check circuit provides the position signal when each pulse of the set consisting of the first pulse, the second pulse, the third pulse, and the fourth pulse, is characterized by the second pulse width within the second range.   
     
     
       18. The servo system of claim 7 wherein: a. the discriminator circuit further comprises a digital to analog converter that provides an analog drive signal in response to the ratio; and   b. the head positioner is further responsive to the analog drive signal for moving the head.   
     
     
       19. The servo system of claim 18 wherein: a. the discriminator circuit further comprises an analog compensation circuit that provides a compensated drive signal in response to the analog drive signal; and   b. the head positioner is further responsive to the compensated analog drive signal for moving the head.   
     
     
       20. The servo system of claim 19 wherein: a. the head positioner provides an analog position signal; and   b. the analog compensation circuit provides the compensated analog drive signal in further response to the analog position signal.